Maastrichtian planktic foraminifera of the South Atlantic Ocean

Stratigraphic, faunal and isotopic analyses of the Maastrichtian at DSDP sites 525A and 21 in the South Atlantic reveal a planktic foraminiferal fauna characterized by two major events, an early late Maastrichtian diversification and end-Maastrichtian mass extinction. Both events are accompanied by major changes in climate and productivity. The diversification event which occurred in two steps between 70.5 and 69.1 Ma increased species richness by a total of 43% and coincided with the onset of major cooling in surface and bottom waters and increased surface productivity. The onset of the terminal decline in Maastrichtian species richness began at 67.5 Ma and the first significant decline in surface productivity occurred at 66.2 Ma, coincident maximum cooling to 13°C in surface waters and the reduction of the surface-to-deep temperature gradient to less than 5°C. Major climatic and moderate productivity changes mark the mass extinction and the last 500 kyr of the Maastrichtian. Between 200 and 400 kyr before the K-T boundary surface and deep waters warmed rapidly by 3-4°C and cooled again during the last 100 kyr of the Maastrichtian. Surface productivity decreased only moderately across the K-T boundary. Species richness began to decline during the late Maastrichtian cooling and by K-T boundary time, the mass extinction had claimed 66% of the species. Viewed within the context of Maastrichtian climate and productivity changes, the K-T mass extinction could have resulted from extreme environmental stress even without the addition of an extraterrestrial impact.

对南大西洋深海钻探计划（Deep Sea Drilling Project, DSDP）525A孔与21孔马斯特里赫特阶（Maastrichtian）开展的地层学、动物群学及同位素分析显示，该区域浮游有孔虫（planktic foraminifera）动物群存在两次重大演化事件：晚马斯特里赫特早期的物种辐射演化，以及马斯特里赫特期末的集群灭绝。这两次事件均伴随气候与古生产力的显著变化。此次辐射演化事件于70.5 Ma至69.1 Ma间分两阶段发生，使物种丰富度整体提升43%，并与表层及底层海水大规模降温事件、表层海水古生产力升高的时期相吻合。马斯特里赫特阶物种丰富度的末期衰减始于67.5 Ma，而表层海水古生产力首次显著下降则发生于66.2 Ma，此时恰好对应表层海水降温至13℃的极端低温事件，以及表深温度梯度缩小至5℃以内的现象。集群灭绝事件及马斯特里赫特阶最后500 kyr的沉积记录中，存在显著的气候波动与中等幅度的古生产力变化。在白垩纪-第三纪（K-T）界线前200~400 kyr期间，表层与深层海水快速升温3~4℃，并在马斯特里赫特阶最后100 kyr内再次降温。白垩纪-第三纪界线前后，表层海水古生产力仅出现中等幅度的下降。物种丰富度在晚马斯特里赫特阶降温阶段便已开始衰减，至白垩纪-第三纪界线时期，集群灭绝已导致66%的物种消失。结合马斯特里赫特阶的气候与古生产力变化背景来看，即便不考虑地外撞击事件的影响，白垩纪-第三纪集群灭绝也可能由极端环境压力所引发。